Officials from the World Health Organization (WHO) have put the world on alert about the Zika virus, which is now “spreading explosively” and could soon affect up to 4 million people around the Americas.

“The level of alarm is extremely high,” WHO general-director Margaret Chan said in a briefing. “Last year the virus was detected in the Americas, where it is now spreading explosively.”

Chan warned that the “wide geographic distribution of the mosquito” that transmits the disease poses a major risk for global health. WHO has decided to convene an emergency committee on the Zika virus to evaluate possible solutions.

A connection has been suggested between the Zika virus and microcephaly, a birth defect that causes infants to be born with abnormally small brains and mental retardation. Although a causal relationship hasn’t been proved yet, Brazil saw its rate of microcephaly increase 20-fold after the virus spread there.

The Zika virus was first identified in Uganda in 1947, but the first known case of infection in South America took place in 2015. The disease has now spread to 23 countries and was confirmed to have reached the U.S. earlier this week.

Los Angeles public health officials confirmed that a woman who traveled to El Salvador had been infected with the virus. Officials have now confirmed at least 31 cases across 11 states and the District of Columbia.

Marcos Espinal, a health analysis director for WHO, said we can expect to see “3 to 4 million cases” the disease, according to the Washington Post.

In a number of countries swamped with Zika cases, the government has recommended that women refrain from getting pregnant and wait out the epidemic. Most notably, El Salvador recommended that its women avoid getting pregnant until 2018.